1. Art Field
This invention relates to a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer stacked on the substrate, and a dielectric layer, a light-emitting layer and a transparent electrode layer stacked on the electrode layer.
2. Background Art
EL devices are now practically used in the form of backlights for liquid crystal displays (LCDs) and watches. An EL device works on a phenomenon in which a substance emits light at an applied electric field, viz., an electro-luminescence (EL) phenomenon. The EL device is broken down into two types, one referred to as a dispersion type EL device having a structure wherein electrode layers are provided on the upper and lower sides of a dispersion with light-emitting powders dispersed in an organic material or porcelain enamel, and another as a thin-film EL device using a thin-film light-emitting substance provided on an electrically insulating substrate and interposed between two electrode layers and two thin-film insulators. These types of EL devices are each driven in a direct or alternating voltage drive mode. Known for long, the dispersion type EL device has the advantage of ease of fabrication; however, it has only limited use thanks to low luminance and short service life. On the other hand, the thin-film EL device has recently wide applications due to the advantages of high luminance and very long-lasting quality.
The structure of a typical double-insulation type thin-film EL device out of conventional thin-film EL devices is shown in FIG. 2. In this thin-film EL device, a transparent substrate 21 formed of a green glass sheet used for liquid crystal displays or PDPs is stacked thereon with a transparent electrode layer 22 comprising an ITO of about 0.2 xcexcm to 1 xcexcm in thickness and having a given striped pattern, a first insulator layer 23 in a transparent thin-film form, a light-emitting layer 24 of about 0.2 xcexcm to 1 xcexcm in thickness and a second insulator layer 25 in a transparent thin-film form. Further, an electrode layer 26 formed of, e.g., an Al thin-film patterned in a striped manner is provided in such a way as to be orthogonal with respect to the transparent electrode layer 22. In a matrix defined by the transparent electrode layer 22 and the electrode layer 26, voltage is selectively applied to a selected given light-emitting substance to allow a light-emitting substance of a specific pixel to emit light. The resultant light is extracted from the substrate side. Having a function of limiting currents flowing through the light-emitting layer, such thin-film insulator layers make it possible to inhibit the dielectric breakdown of the thin-film EL device, and so contribute to the achievement of stable light-emitting properties. Thus, the thin-film EL device of this structure has now wide commercial applications.
For the aforesaid thin-film transparent insulator layers 23 and 25, transparent dielectric thin films of Y2O3, Ta2O5, Al3N4, BaTiO3, etc. are formed at a thickness of about 0.1 to 1 xcexcm by means of sputtering, evaporation or the like.
For light-emitting materials, ZnS with yellowish orange light-emitting Mn added thereto has mainly been used due to ease of film formation and in consideration of light-emitting properties. For color display fabrication, the use of light-emitting materials capable of emitting light in the three primary colors, red, green and blue is inevitable. These materials known so far in the art, for instance, include SrS with blue light-emitting Ce added thereto, ZnS with blue light-emitting Tm added thereto, ZnS with red light-emitting Sm added thereto, CaS with red light-emitting Eu added thereto, ZnS with green light-emitting Tb added thereto, and CaS with green light-emitting Ce added thereto.
In an article entitled xe2x80x9cThe Latest Development in Displaysxe2x80x9d in xe2x80x9cMonthly Displayxe2x80x9d, April, 1998, pp. 1-10, Shosaku Tanaka shows ZnS, Mn/CdSSe, etc. for red light-emitting materials, ZnS:TbOF, ZnS:Tb, etc. for green light-emitting materials, and SrS:Cr, (SrS:Ce/ZnS)n, Ca2Ga2S4:Ce, Sr2Ga2S4:Ce, etc. for blue light-emitting materials as well as SrS:Ce/ZnS:Mn, etc. for white light-emitting materials.
IDW (International Display Workshop), ""97 X. Wu xe2x80x9cMulticolor Thin-Film Ceramic Hybrid EL Displaysxe2x80x9d, pp. 593-596 shows that SrS:Ce out of the aforesaid materials is used for a thin-film EL device having a blue light-emitting layer. In addition, this publication shows that when a light-emitting layer of SrS:Ce is formed by an electron beam evaporation process in a H2S atmosphere, it is possible to obtain a light-emitting layer of high purity.
However, a structural problem with such a thin-film EL device remains unsolved. The problem is that since the insulator layers are each formed of a thin film, it is difficult to reduce to nil steps at the edges of the pattern of the transparent electrode, which occur when a large area display is fabricated, and defects in the thin-film insulators, which are caused by dust, etc. occurring in the process of display production, resulting in a destruction of the light-emitting layer due to a local dielectric strength drop. Such defects offer a fatal problem to display devices, and produce a bottleneck in the wide practical use of thin-film EL devices in a large-area display system, in contrast to liquid crystal displays or plasma displays.
To provide a solution to the defect problem with such thin-film insulators, JP-A 07-50197 and JP-B 07-44072 disclose a thin-film EL device using an electrically insulating ceramic substrate as a substrate and a thick-film dielectric material for the thin-film insulator located beneath the light-emitting substance. As shown in FIG. 3, this thin-film EL device has a structure wherein a substrate 31 such as a ceramic substrate is stacked thereon with a lower thick-film electrode layer 32, a thick-film dielectric layer 33, a light-emitting layer 34, a thin-film insulator layer 35 and an upper transparent electrode 36. Unlike the thin-film EL device shown in FIG. 2, the transparent electrode layer is formed on the uppermost position of the device because the light emitted from the light-emitting substance is extracted out of the upper side of the device facing away from the substrate.
The thick-film dielectric layer in this thin-film EL device has a thickness of a few tens of xcexcm to a few hundred xcexcm or is several hundred to several thousand times as thick as the thin-film insulator layer. Thus, the thin-film EL device has the advantages of high reliability and high fabrication yields because of little or no dielectric breakdown caused by pinholes formed by steps at electrode edges or dust, etc. occurring in the device fabrication process. The use of this thick-film dielectric layer leads to another problem that the effective voltage applied to the light-emitting layer drops. However, this problem can be solved or eliminated by using a high dielectric constant material for the dielectric layer.
However, the light-emitting layer stacked on the thick-film dielectric layer has a thickness of barely a few hundred nm that is about {fraction (1/100)} of that of the thick-film dielectric layer. For this reason, the thick-film dielectric layer must have a smooth surface at a level less than the thickness of the light-emitting layer. However, it is still difficult to sufficiently smooth down the surface of a dielectric layer fabricated by an ordinary thick-film process.
To be more specific, a thick-film dielectric layer, because of being essentially constructed of ceramics using a powdery material, usually suffers from a volume shrinkage of about 30 to 40% upon closely sintered. However, ordinary ceramics are closely packed through a three-dimensional shrinkage upon sintering whereas a thick-film ceramic material formed on a substrate does not shrink across the substrate because the thick film is constrained to the substrate; its volume shrinkage occurs in the thickness direction or one-dimensionally alone. For this reason, the sintering of the thick-film dielectric layer does not proceed to a sufficient level, yielding an essentially porous layer.
Since the process of close packing proceeds through a ceramic solid phase reaction of powders having a certain particle size distribution, sintering abnormalities such as abnormal crystal grain growth and macropores are likely to occur. In addition, the surface roughness of the thick film is absolutely greater than the crystal grain size of polycrystal sintered grains and, accordingly, the thick film has surface asperities of at least sub-xcexcm size even though it is free from such defects as mentioned above.
When the dielectric layer has surface defects or a porous structure or asperity shape as mentioned above, it is impossible to deposit thereon a light-emitting layer formed by evaporation, sputtering or the like uniformly following the surface shape thereof. This makes it impossible to effectively apply an electric field to the portion of the light-emitting layer formed on a non-flat portion of the substrate, resulting in problems such as a decrease in the effective light-emitting area, and a light emission luminance decrease due to a local dielectric breakdown of the light-emitting layer, which is caused by local non-uniform thicknesses. Furthermore, locally large thickness fluctuations cause the strength of an electric field applied to the light-emitting layer to vary too locally largely to obtain any definite light emission voltage threshold.
Thus, operations for polishing down large surface asperities of a thick-film dielectric layer and then removing much finer asperities by a sol-gel step are needed for conventional fabrication processes.
However, the polishing of a large-area substrate for display or other purposes is technically difficult to achieve, and is a factor for cost increases as well. The addition of the sol-gel step is another factor for cost increases. When a thick-film dielectric layer has abnormal sintered spots which may give rise to asperities too large for removal by polishing, yields drop because they cannot be removed even by the addition of the sol-gel step. It is thus very difficult to use a thick-film dielectric material to form a light emission defect-free dielectric layer at low cost.
A thick-film dielectric layer is formed by a ceramic powder material sintering process where elevated firing temperature is needed. As is the case with ordinary ceramics, a firing temperature of at least 800xc2x0 C. and usually 850xc2x0 C. is needed. To obtain a closely packed thick-film sintered body in particular, a firing temperature of at least 900xc2x0 C. is needed. In consideration of heat resistance and a reactivity problem with respect to the dielectric layer, the substrate used for the formation of such a thick-film dielectric layer is limited to alumina or zirconia ceramic substrate; it is difficult to rely on inexpensive glass substrates. The requisite for the aforesaid ceramic substrate to be used for display purposes is that it has a large area and satisfactory smoothness. The substrate meeting such conditions is obtained only with much technical difficulty, and is yet another factor for cost increases.
For the metal film used as the lower electrode layer, it is required to use costly noble metals such as palladium and platinum in view of its heat resistance. This, too, is a factor for cost increases.
An object of the invention is to provide a solution to all problems with conventional thin-film EL devices, viz.,
(1) an fatal defect to display devices, which, when an insulator layer is made up of a thin film, results from destruction of a light-emitting layer due to a local dielectric strength decrease ascribable to defects in the insulator layer,
(2) poor light emission properties which, when a thick-film dielectric layer made up of ceramics is used, result from defects on the surface of the dielectric layer, the fact that the dielectric layer is porous, and the asperity configuration of the surface of the dielectric layer,
(3) cost increases due to the addition of a difficult-to-perform step, i.e., the step of polishing the surface of the thick-film dielectric layer, and further cost increases due to the addition of a sol-gel step, and
(4) limitations imposed on the selection of substrate and electrode layer materials thanks to the firing temperature for the thick-film dielectric layer.
Another object of the invention is to provide a thin-film EL device and its fabrication process without incurring any cost increase. The thin-film EL device of the invention allows restrictions on the selection of substrate materials to be removed, and so makes it possible to use glass substrates which are inexpensive and can be easily processed to a large area, and can rely on a quick-and-easy process to make correction for non-flat portions of a dielectric layer due to an electrode layer, and dust, etc. resulting from process steps, thereby preventing any dielectric strength decrease. In addition, the thin-film EL device of the invention ensures high display quality by virtue of the satisfactory flatness of the surface of the dielectric layer.
The aforesaid object is achievable by the following embodiments (1) to (5) of the invention.
(1) A thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer stacked on said substrate, and a dielectric layer, a light-emitting layer and a transparent electrode stacked on said electrode layer, wherein:
said dielectric layer is a multilayer dielectric layer formed in a multilayer form by repeating a solution coating-and-firing step plural times, and
said multilayer dielectric layer has a thickness of at least four times as large as a thickness of said electrode layer and 4 xcexcm to 16 xcexcm inclusive.
(2) The thin-film EL device according to (1) above, wherein said multilayer dielectric layer is formed by repeating said solution coating-and-firing step at least three times.
(3) The thin-film EL device according to (1) above, wherein said multilayer dielectric layer has a thickness per sub-layer of at least xc2xd of said electrode layer.
(4) A process of fabricating a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer stacked on said substrate, and a dielectric layer, a light-emitting layer and a transparent electrode stacked on said electrode layer, wherein:
said dielectric layer is provided on said electrode layer in a multilayer form by repeating coating-and-firing of a dielectric precursor solution plural times.
(5) The process of fabricating a thin-film EL device according to (4) above, wherein the coating-and-firing of said dielectric precursor solution is repeated at least three times.